Proceedings of the 2013 International Conference on Advanced Mechatronic Systems, Luoyang, China, September 25-27,2013

An Improved Logic Threshold Approach of Energy Management for a Power-Split Hybrid Electric

Vehicle

Zhu-mu FU 1,2 *

ISchool of Control Science and Engineering, Shandong University, Jinan250061, China

*Corresponding author Email: fzmI974@163.com

Abstract-The power-split hybrid electric vehicle (HEY) has

the unique advantages of the series and parallel types of HEY

power train configurations. However, it needs a sophisticated

control system to manage the power-split HEY power trains. To

simplify control strategy and improve fuel-saving capability, we

design an improved logic threshold approach of energy

management for a power-split HEY assisted by an integrated

starter generator (ISG). The improved logic threshold controller

manages the ICE within its peak-efficiency region at first. Then

the electrical power demand is established based on the ICE

energy output. On that premise, a variable logic threshold value

is defined to achieve the power distribution between the ISG and

the electric motor/generator (EMG). Finally, simulation models

for the power-split HEY with improved logic threshold controller

are established in ADYISOR. The ICE, EMG and ISG operating

performances of power-split HEY are performed under two

driving cycles. The comprehensive results show that battery

power consumption reduces up to 8%, the ICE efficiency

improves up to 1.7%, the motor driving system efficiency

improves up to 6.1 % of a power-split HEY with the improved

logic threshold approach compared with one with the logic

threshold approach.

Keywords-power-split HE V; ISG; logic threshold approach

1. INTRODUCTION

To improve the efficiency and fuel economy of hybrid electric vehicle (HEV), many researchers focus on power-split HEVs [1, 2] as they can achieve a potential of higher fuel economy and reduce the electrical system loses. The power­split hybrid system which usually uses an ICE with integrated starter generator (TSG) [3] and an electric motor/generator (EMG), combines benefits of both the parallel and series type hybrid systems without the cost effectiveness of this hybrid system.

Since the structure of power-split HEV is more complicated [2], [4], it needs a sophisticated control system to manage the power-split HEV power trains. Such a control system requires a reasonable control strategy to improve the fuel-saving capability of the ICE at first [5, 6]. To solve this problem, studies such as logic threshold approach applied to the design of the control strategy for HEV have been used to achieve the power or torque distribution [7]. However, due to individual characters of the power-split HEVs [2], and even different

This project was supported by China Postdoctoral Science Foundation under Grant 2013T60670, Science and Technology Programme Foundation for the Innovative Talents of Henan Province University under Grant 13HASTIT038 and the Key Scientific and Technological Project of Henan Province under Grant 132102210247.

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Bin WANG 2, Peng-ge ZHOU 2 2Electronic & Information Engineering College, Henan University of Science and Technology,

Luoyang 471023, China

driving performance under different drive cycles to the same power-split HEV[8, 9], it is necessary to consider the individual ICE, structure and other driving conditions such as SOC, vehicles' speed of the HEV when designing the control strategy [10].

Tn this case, by analyzing the structure of a power-split HEV and its operation, we have designed an improved logic threshold controller to achieve the power distribution in the ICE and electrical system. Furthermore, to obtain the actual torque distributed in electrical system, a variable logic threshold value K is defmed to achieve the power distribution between the TSG and the EMG. The improved logic threshold controller behaviors in simulation under different driving conditions are compared with the performances with logic threshold controller system. Results clearly demonstrate that the improved logic threshold approach can significantly improve the efficiency of the ICE and electrical system behavior.

II. STRUCTURE AND OPERATION OF POWER-SPLIT HEV

The structure of power-split HEV is shown in fig. 1. To reduce inefficiencies of a single motor drive system, the power­split HEV adopts two motors in its electrical system. That is the TSG and the EMG. The TSG is integrated with the TCE by a gear set. The EMG is downsized and integrated with the ISG and the TCE. Both the TSG and the EMG can work as drive motor and generator. Since this structure increases the complexity of the electrical system, an electric power splitter and a central power splitter are used in it. To insure the independences of the TCE, the TSG and the EMG, it adds four clutches.

Gear set as the

ICE

Batteries

Central power

Electric splitter

Fig. I Structure of power-split HEY

When driving the power-split HEV, ICE optimum torque curve can be used as a logic threshold value, TCE is operating at the optimum torque curve. By neglecting energy losses, the relationship between the TCE, the TSG and the EMG can be expressed as

{k T = T = Treq -Tor" Tor" > Treq > 0 1 1Sg elc Ice Ice ' Ice u;e kl'0,g + k2 �mg = �le = '0::q -'0:�)/ , else

{ = k = k TOP' oF 0 OJisg 1 Wice , OJemg 2 Wice , ice k - k TOp, - 0 2 Wisg - IOJemg, ice -

(1)

(2)

where '0SI( is the output torque from the TSG; �/e is the output

torque of the electrical system; r;;;q is the torque demand on

the ICE; r;:�" is the optimum output torque of the ICE, when

the TCE is stopped, r;::" = 0 ; �mg is the output torque from the

EMG; k[ and k2 are the gear ratios; OJ;SI( is the speed demand

on the ISG; OJ;ce is the actual speed of the ICE, which depends

on the power-split HEV speed and the gear ratios; OJemg is the

speed demand on the EMG.

Equations (1) and (2) reflect the relationship between the TCE, the TSG and the EMG when driving the power-split HEV. It is easy to calculate the power distribution of every component on the basis of obtaining torque and speed. For instance, the optimum output power p;:�)/ from the ICE can be

calculated by using '0:;' and optimum output speed OJi"::' of the

TCE.

(3)

By combining (1) and (3), the output power of the ICE can be controlled at highest efficiency by changing the output torque of the ISG or the EMG. Since p;��' is known, the power

demand on electrical system can be obtained by

{p = preq _ por" '."C u.:

.e

. Ice

preq = Treq OJ Ice Tee Ice (4)

Where �l' is the power demand on electrical system, p;::q is

the power demand on the TCE.

It can distribute the power between the ISG and the EMG in the electrical system by knowing �1' . At a certain speed,

when �le is less than the peak-power of the TSG, the electrical

power is supplied by the TSG. When �1' is up to the peak­

power of the TSG and less than the peak-power of the EMG, the electrical power is supplied by the EMG. When �1' is up

to the peak-power of the EMG, the electrical power is supplied by the ISG and the EMG.

It can be seen the power distribution between the TCE, the ISG and the EMG is very easy when using simple logic threshold approach. However, the TCE may fail to achieve the best efficiency due to the complex nature of power-split HEV.

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For example, the inertial additional torque of the ICE, the SOC of batteries and the drive cycles are usually important factors to affect the ICE output. 1n this condition, the ICE is operating at region near the optimum torque curve. The relationship between the ICE, the ISG and the EMG can be described as

{k T = T = Treq - TaCl TOP' > Treq > 0 1 1Sg elc Ice Ice 'Tee Ice k T + k T - T - req -re' I 1 isg 2 emg -ele -ice ice ,e se

where r;;;' is the actual output torque of the ICE, r;;;' :?: 0 .

(5)

Equation (5) reflects the actual torque distribution among the ICE, the ISG and the EMG. One can see the logic threshold value '0:�)/ is very important to design the highest efficiency of

the ICE. Because the threshold value r;:�' is changed into the

actual output torque, the control strategy fails to achieve the best efficiency of the TCE. Since simple logic threshold approach isn't available to achieve the best efficiency of the TCE, we propose an improved logic threshold approach of energy management by taking into account the factors such as the character of (TCE), the SOC and the speed of power-split HEV.

TIT. DESIGN OF THE IMPROVED LOGIC THRESHOLD

APPROACH

The TCE logic threshold controller calculates the torque distribution to the ICE. To calculate r;;;' , it is very important

to consider the SOC of batteries at first because the SOC is related to the ICE output torque. The resistance curves and the voltage curve corresponding to SOC of the single NI-MH battery used in this study are shown in fig.2. To ensure batteries charging-discharging efficiency, we define that when SOC is upon to 0.8, the ICE must be stopped to avoid over charge of batteries. When SOC is less than 0.2, the TCE must be started to avoid over discharge of batteries.

discharg e

0.2 0.4 0.6 0.8 1.0 SOC

Fig. 2 The resistance curves corresponds to SOC

When SOC is in the region [0.2 0.8], the start/stop of the ICE is determined by a binary variable. The binary variable is expressed as follows

{I, 0.2::; soc::; 0.5 e(t-l)=O,e(t)=

0, else (6)

{O, 0.5 ::; SOC::; 0.8 e(t-I) = I,e(t) =

1, else (7)

Where e(t) is the binary variable, e(t) = 0 indicates off status

of the ICE, the actual output torque is 0; e(t) = 1 indicates that

the ICE is started.

We can see 0.5 can be used as a logic threshold value of SOC to change the state of ICE. However, the state of ICE may be changed quite frequently when SOC is changed in regions around SOC = 0.5 . So the single logic threshold value of SOC is improved, and the single value is changed to a region [0.45, 0.55]. In this region, the state of the ICE is maintained, the binary variable is expressed as follows

{I, 0.2::; SOC::; 0.45 e(t -1) = O,e(t) = 0, else

{0,0.55::; SOC::; 0.8 e(t-l) = l,e(t) =

1, else

(8)

(9)

From (8) and (9), we can see the batteries should be charged when the SOC is lower than 0.45, and be discharged when SOC is more than 0.55. The ICE provides the charge or discharge power, so the actual output torque of the ICE 7;::' must be related to the value of SOC and the modes of the vehicle.

When e(t) = 0 , it can be known that 7;;;' = 0 because the

ICE is turned off. When e(t) = 0 , 7;;;' > 0 , the calculation of

'0:;' should be determined by the modes of the vehicle. The

modes of the vehicle are determined by I'!.T here.

(10)

a) When I'!.T > 0 and '0;;q < TL(w) : Operating the ICE

within the low torque region is uneconomical. It is avoidable by activating the recharging mode so that the ICE can operate in its peak-efficiency. '0::' can be calculated as {Tac,

= T + 0.45 -SOC (TO'!l _ T ) SOC < 0 4 ice

.

L(w) 0.45 _ 0.2 ice L(w) , - . 5

'0::' = '0(W)' 0.45 < SOC < 0.55

(11)

b) When I'!.T < 0 and 7;;;q > TH(w) , the ICE is operated

at the highest output torque by activating the hybrid mode. '0::' is limited as

Tact = T Ice H(m) (12)

c) When I'!.T = 0 and '0(W) < 7;;;g < TH(w) , since operating

the ICE alone within the peak-efficiency region is economical, the pure ICE mode is activated '0::' can be calculated as

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{Tact = rup' 0 45 < SOC < 0 5

;:ct = ;::q

'+

·0.45 -SOC (; _ reg) SOC::; 0.45 ,ce ,ce 0.45 _ 0.2

H(w) ,ce' (13)

When we get 7;::' , we should calculate the electrical

system power demand �le. The relation of the power between

the ICE and the electrical system can be expressed by rewriting (4) as follows:

{p =

preg _ pact eie Ice Ice �::t

= �::t OJice (14)

To distribute the torque between the ISO and the EMO, we defme a variable logic threshold value K to achieve the distribution, which is shown as follows

K = P;sg / �/e (15)

Where K = {O, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, I}, P;sg is

the power contribution of the ISO to the electrical power demand �/e . The power contribution of the EMO can be

calculated as

(16)

Where �mg is the power contribution of the EMO.

The efficiency of electrical system can be calculated as

Where 17:,g is the efficiency of the ISO; 17:"," is the efficiency of

the EMO; i = I , the ISO and the EMO work in recharge state; i = -1 , they work in discharge state.

By combining (15)-(17), the best efficiency 17e/emax of

electrical system can be calculated as { - {K ' + (1-K); }' 17e/e - 17"g 17emg

, 17e/emax = arg max

. {K17:,g +(l-K)17;mg}

Kc (Kmm.Kmox)

(18)

For a particular K , P;sg and �mg can be calculated by (15) and

(16). On the other hand, OJ;sg and OJel1lg can be obtained by

knowing the wheel speed corresponding to drive cycles,

17' a and 17' by the calculation ISr, "'III);

{ 17:'1( = f ('0SI(' OJ;\!(), '0 sg = P;SI( / OJ;SI(

17:mg = f(�l1Ig' OJel1lg), �ml( = �ml( / OJeml( (19)

From (18) and (19), we can get the efficiency of electrical system to any particular K . At the same time, we can get the maximum efficiency 17e/emax by searching the maximum value of

lJe/e , then the value K corresponding to lJe/emax IS used to

calculated p;\g and �mg

IV. SIMULATION AND COMPARATIVE ANALYSIS

The Urban Dynamometer Driving Schedule (UDDS) and the New European Drive Cycle ( NEDC) are chosen to demonstrate the improved logic threshold approach. The important parameters of power-split HEV are listed in table I . The selected drive cycles are shown in Fig.3.

TABLE 1. PARAMETERS OF VEHICLES

component

ICE

(Honda­

Insight)

EMG

(Insight)

ISG

(Insight)

NIHM battery

power-split

HEV data

100

00-E ::,c

as 50 Q) Cl.

(f)

o

parameter

Peak power

Optimum torque

Peak efficiency

Peak power

Peak torque

Peak power

Peak torque

voltage

capacity

Radius of wheel

Frontal area

Total mass

200

Power-split HEV value

50kw

60Nm

0.4

30kw

± 150Nm

10kw

± 50Nm

288V

6.5Ah

0.275m

1.92 m2

1350Kg

600 800 1000 1200 1400 Time (5)

a) NEDC

FigA shows the comprehensive results of the power-split HEV in NEDC. The torque outputs of the ICE, the ISO and the EMG reflect that the ICE controlled by both logic threshold controller and improved logic threshold controller can work in the peak-efficiency region. When the electrical system responds to high regenerative braking toque, the ISO and the EMG controlled by improved logic threshold controller can work in more harmony than logic threshold controller since the logic threshold variable K is used to achieve the torque distribution between the ISG and the EMG. On the other hand, it reflects the change of SOC, the initial value of the SOC is set to 0.7. The [mal value is 0.51 when the logic threshold controller is used, but the final value is 0.59 when the improved logic threshold controller is used. The improved logic threshold controller reduces the energy consumption of batteries up to 8%.

To have more specific comparative analysis, we list the ICE efficiency, the variation of the SOC and the motor drive system efficiency calculated from ADVISOR under the two driving cycles shown in table II. We can see the improved logic threshold controller improves the ICE efficiency and reduces the variation of the SOC. The motor drive system efficiency is improved apparently. The ICE efficiency is increased by 1.7% and the motor drive system efficiency is increased by 6.1% when using the improved logic threshold controller, compared with the ICE with the logic threshold controller.

80

b) UDDS

Fig. 3 Driving cycles

Time (s) Time (s)

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(a) Comprehensive results with logic threshold (b) Comprehensive results witb improved logic tbreshold

Fig.4 Comprehensive results in NEDC

TABLE I!. COMPARISON OF SIMULATION RESULTS

ICE Efficiency The variation of the SOC

motor driving system efficiency driving (initial SOC- final SOC) cycle improved logic logic logic threshold threshold threshold

NEDC 35.8% 37.6% 0.70-0.51

UDDS 34.7% 36.4% 0.70-0.47

V. CONCLUSIONS

This paper designs an improved logic threshold approach of energy management for a power-split HEV assisted by an ISO. By analyzing the power-split HEV structure and its operation, the improved logic threshold controller combines the ICE characters, the demand torque and the value of the SOC to manage the ICE within its peak-efficiency region. Furthermore, a variable logic threshold value K is defmed to achieve the best efficiency of electrical system and the power distribution between the ISO and the EMO.

The comprehensive results show that battery power consumption reduces up to 8%, the ICE efficiency improves up to 1.7%, the motor driving system efficiency improves up to 6.1 % of a power-split HEV with the improved logic threshold approach compared with one with the logic threshold approach. The comprehensive results show the effectiveness and the validity of the improved logic threshold approach.

ACKNOWLEDGMENT

The authors would like to thank Henan University of Science and Technology for material support. This project was supported by China Postdoctoral Science Foundation (No. 2013T60670) and Science and Technology Programme Foundation for the Innovative Talents of Henan Province University (No. 13HASTlT038) and supported by Key Scientific and Technological Project of Henan Province (Orant No. 132102210247 and 102102210449).

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